1. Field of the Invention
The invention relates to circuits and methods of transforming a mono-band antenna into a multi-band antenna and more particularly to transforming a mono-band antenna by adding impedance matching circuits for two or more frequency bands which can range from a few megahertz to several hundred megahertz.
2. Description of the Related Art
At present small aperture antennas are being used in small size, short range wireless devices such as keyboards, mice, alarm systems and other similar devices. These devices are used for low frequencies extending from a few megahertz to several hundred megahertz. Such an antenna consists of a small size loop and a mono-band matching circuit. These loops are usually printed on a PCB (Printed Circuit Board) but they can also be made of wire. Recently, demand for multi-band operation has appeared. The reason is the need of interference avoidance and multi task applications. Separate antennas and/or PIN (positive-intrinsic-negative) diode switching are not good solutions because of the requirement for small size and low power consumption. The proposed multi-band antenna is ideal for such applications.
FIG. 1 shows a typical mono-band loop antenna. Inductance L and resistance R represent the antenna loop model, i.e. L and R model the antenna, where capacitor C1 sets the resonant frequency and capacitor C2 sets the matching impedance of the antenna. The diamond symbol indicates the signal input and is labeled Port 1. The numbers next to L, R, C1 and C2 are typical values for those network elements.
FIG. 2 is a variation of FIG. 1, where capacitor C1 is moved to the other side of the antenna and is grounded for better performance.
U.S. patents or U.S. patent application Publications which relate to the present invention are: U.S. Pat. No. 6,795,714 (Fickenscher et al.) discloses a multi-band antenna switch which can be used for switching between a branch and a receiving branch of a multi-band mobile radio telephone. This invention switches the transmitting and receiving branches of the mobile radio telephone so as to be differentiated from one another in time. U.S. Patent Application Publication 2006/0028332 (Miller) describes a multi-band antenna which is adapted to receive multiple RF signals. A multiplexer, comprising L-C resonant circuits attached to the antenna, separates and distributes the incoming frequencies to different users.
It should be noted that in the above-cited examples of the related art multiple frequencies are applied to multiple ports. None of the above-cited examples of the related art provide multiple frequencies applied to a single port. Accordingly, a new approach is desirable where multiple frequencies can be transmitted by a single antenna from a single port.